Ultrathin aluminum oxide films: Al-sublattice structure and the effect of substrate on ad-metal adhesion

First principles density-functional slab calculations are used to study 5 Å (two O-layer) Al2O3 films on Ru(0001) and Al(111). Using larger unit cells than in a recent study, it is found that the lowest energy stable film has an even mix of tetrahedral (t) and octahedral (o) site Al ions, and thus most closely resembles the κ-phase of bulk alumina. Here, alternating zig-zag rows of t and o occur within the surface plane, resulting in a greater average lateral separation of the Al-ions than with pure t or o. A second structure with an even mix of t and o has also been found, consisting of alternating stripes. These patterns mix easily, can exist in three equivalent directions on basal substrates, and can also be displaced laterally, suggesting a mechanism for a loss of long-range order in the Al-sublattice. While the latter would cause the film to appear amorphous in diffraction experiments, local coordination and film density are little affected. On a film supported by rigid Ru(0001), overlayers of Cu, Pd, and Pt bind similarly as on bulk truncated α-Al2O3(0001). However, when the film is supported by soft Al(111), the adhesion of Cu, Pd, and Pt metal overlayers is significantly increased: oxide-surface Al atoms rise so only they contact the overlayer, while substrate Al metal atoms migrate into the oxide film. Thus the binding energy of metal overlayers is strongly substrate dependent, and our numbers for the above Pd-overlayer systems bracket a recent experimentally derived value for a film on NiAl(110).